1. Field of the Invention
This invention relates to a method for electrically connecting electrodes of a semiconductor device (chip, pellet or die) to terminal electrodes on a circuit board. More specifically, the invention relates to a method for mounting a semiconductor device by face down bonding using metallic paste for connection, or a method for mounting a semiconductor device which involves the use of solder bumps for electrical connection of electrodes; a mounted structure of a semiconductor device produced by any of these methods; and a mounting system for the methods and the mounted structure.
2. Description of the Related Art
Soldering has been generally used for electrical connection of connecting electrodes of an electronic device to circuit pattern terminals on a circuit board. In recent years, a small package, such as an IC flat package, has come into use, and the number of connecting terminals has increased. Thus, a so-called pitch between adjacent connecting terminals has gradually narrowed, and it has become gradually difficult for a conventional soldering technique to provide electrical connections stably in such a narrow pitch.
To mount a semiconductor device, such as a chip, pellet or die, which is an uncovered active or passive device, called a bare device, on a circuit board while establishing an electrical connection between them, face down bonding has recently been in wide use. The face down bonding method comprises forming solder bumps beforehand on electrode pads of the semiconductor device, placing the solder bumps so as to downwardly face terminal electrodes on the circuit board, and heating the solder bumps at a high temperature to fusion bond the electrode of semiconductor device to the terminal electrode of circuit board. The solder bumps are generally formed on three metallic thin films (under bump metals) comprising, for example, Cr (chromium), Cu (copper) and Au (gold), by soldering, plating or vapor deposition using a resist pattern.
This mounting method imparts high mechanical strength after connection, and is capable of electrically connecting the plural electrodes of the semiconductor device to the terminal electrodes of the circuit board at a time. Thus, it has been considered an effective method for mounting a semiconductor device.
In the conventional method of mounting a semiconductor device using solder bumps, however, the three metallic thin films (under bump metals) comprising, for example, Cr (chromium), Cu (copper) and Au (gold) need to be formed prior to the formation of the solder bumps. Formation of these thin films has required not only considerably extensive equipment, but also mask management and vacuum management, thus leading to increase in the cost and working time. According to the conventional method of mounting a semiconductor device using solder bumps, moreover, the solder spreads during heat fusion, and the adjacent solder bumps (electrodes) may form short-circuit. Thus, this method may fail to be applied to micro-fabricated products.
It has been proposed to form balls from a metallic fine particle paste containing metallic ultra-fine particles, and use these balls instead of the above-mentioned solder bumps (see Japanese Laid-open Patent Publication No. 326416/1997). However, the metallic ultra-fine particles used in this method are considered to be ultra-fine particles of a metal alone which were prepared, for example, by the following method: A metal is evaporated in a vacuum in the presence of a small amount of gas to coagulate ultra-fine particles consisting only of metal from a vapor phase, thereby obtaining ultra-fine metallic particles. Such metallic ultra-fine particles may be problematical in stability, physical properties, and cost.
The present invention has been accomplished in light of the foregoing circumstances. An object of the present invention is to provide a method for mounting a semiconductor device, the method enabling the solder bump method to be conveniently performed, and being capable of realizing a highly stable, low-cost, high reliability electrical connection, and also provide a mounted structure by this method. Another object of the invention is to provide a method for mounting a semiconductor device, the method being capable of realizing a highly stable, low-cost, high reliability electrical connection, which has no risk of short-circuiting adjacent electrodes of even a narrow pitch; and a mounted structure of the semiconductor device produced by this method.
According to the present invention, there is provided a method, a structure and a system for mounting a semiconductor device by the bump technique using compound metallic ultra-fine particles each comprising a core portion consisting substantially of a metallic component, and a coating layer chemically bound to the core portion and comprising an organic substance. The method, the structure and the system are characterized by using one of, or a combination of, the following two bump technologies:
1) Forming under bump metals from the compound metallic ultra-fine particles, and forming ordinary solder balls on the under bump metals.
2) Using paste balls comprising the compound metallic ultra-fine particles, instead of ordinary solder balls.
That is, an aspect of the present invention is a method for mounting a semiconductor device, comprising: preparing a metallic ultra-fine particle paste by dispersing compound metallic ultra-fine particles in a solvent, the compound metallic ultra-fine particles each comprising a core portion consisting substantially of a metallic component and a coating layer chemically bound to the core portion and comprising an organic substance; forming ultra-fine particle electrodes by attaching the metallic fine particle paste onto electrodes of the semiconductor device, followed by low temperature heat process; forming solder bumps on the ultra-fine particle electrodes; and connecting the solder bumps to terminal electrodes of a circuit board by heat fusing thereof.
According to the above methods, the metallic fine particle paste is adhered onto the electrodes of the semiconductor device, and subjected to low temperature heat process. Thus, the ultra-fine particle electrodes, which serve as under bump metals for solder bumps, can be formed easily and promptly without using extensive equipment.
Another aspect of the present invention is a method for mounting a semiconductor device, comprising: preparing a metallic fine particle paste by dispersing compound metallic ultra-fine particles in a solvent, the compound metallic ultra-fine particles each comprising a core portion consisting substantially of a metallic component, and a coating layer chemically bound to the core portion and comprising an organic substance; forming metallic fine particle paste balls consisting mainly of the compound metallic ultra-fine particles by attaching the metallic fine particle paste onto terminal electrodes of a circuit board; connecting electrodes of the semiconductor device onto the metallic fine particle paste balls by the face down bonding method; and electrically connecting electrodes of the semiconductor device and the circuit board by low temperature heat process.
The compound metallic ultra-fine particles can be prepared by a chemical process in a liquid phase. Thus, they can be mass produced in an ordinary atmospheric state, for a low cost, with the use of a simple apparatus without the need to use an extensive apparatus. Furthermore, the compound metallic ultra-fine particles have an organic substance as the coating layer. Thus, when in the solvent, these particles are minimally aggregatable, stable and easy to handle. Consequently, the metallic fine particle paste having the compound metallic ultra-fine particles uniformly dispersed therein can be prepared, and process control for the metallic fine particle paste is easy. Moreover, the particle diameters are uniform, so that all the compound metallic ultra-fine particles are fused and bound together at a constant temperature during low temperature heat treatment.
In the above methods for mounting a semiconductor device, the core portion is a positively charged, metallic ultra-fine particle of Ag, Au or Pb with an average particle diameter of 1 to 10 nm, and the coating layer is an organic anion having 5 or more carbon atoms.
In the methods for mounting a semiconductor device, the low temperature heat process is performed in a temperature range of 200 to 300xc2x0 C. The temperature is not lower than the temperature at which the coating layer is released from the core portion and core portions are fused and bound together.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.